In data acquisition systems for seismic exploration, it is known to transmit signals between a central control system and a plurality of seismic data acquisition devices, which devices are set up to transmit back to the central control system by radio or cable the seismic signals which they have collected, numbered and recorded. These data signals are provided to the acquisition devices by sets of seismic sensors which are distributed along a seismic profile to be studied in response to the echoes of seismic signals generated in the ground by an appropriate shock source on underground reflective layers.
A device which is designed to transmit commands and data by cable or radio waves is described in published French Pat. No. 2,511,772 which corresponds to our above-referenced U.S. Pat. No. 4,583,206. Such device is associated, on the one hand, with the central command and recording system and with a number of acquisition devices, each of which is designed to communicate with the central control and recording system by radio or cable and includes a number of stationary means of connection which make it possible, when they are set up, to connect each device to a common transmission cable and to disconnect the associated means of radio transmission and reception in the device. This makes it possible for the central control and recording system to communicate with each acquisition device either by cable or by radio depending on the type of link actually being used. These acquisition devices associated with their means of transmission and reception by radio and their particular stationary means of connection are housed in boxes (or buoys in the case of underwater exploration) deployed in the field a certain distance apart and connected to the seismic receivers.
After the acquisition devices have collected the data provided by the seismic sensors with which they are associated, the central control and recording system sends transmission commands in succession to all of the acquisition devices in order to have them transmit the signals collected on the common transmission cable or by radio. The data transmissions are preferably made by radio when the transmission quality is adequate. However, when the terrain where the boxes are deployed is rugged or when obstacles (thick forests, for example) make good transmission of the radio waves from certain boxes or certain particular groups of boxes difficult, these boxes should be connected to the central control and recording system by a transmission cable while the others are connected to the central system by radio.
The cable links are very reliable regardless of the nature or relief of the terrain, but the installation of the cables requires time-consuming and expensive laying and removal operations, especially in mountainous or swampy areas.
The selective connection of the boxes or buoys to the common transmission cable makes it possible to significantly simplify the phase of installing the seismic data acquisition system in the field. However, this mixed transmission system sometimes exhibits certain drawbacks when, for example, the number of boxes for which satisfactory radio links are impossible is relatively small and when it is still necessary to deploy a transmission cable, sometimes over long distances, in order to connect certain boxes to the central system while traversing areas where radio links are satisfactory. This case arises, for example, when some of the boxes are deployed on the same side of a terrain elevation as the central control and recording system and is accessible by radio, while other boxes are installed on the opposite side and are accessible only by transmission cable. Such a cable must consequently be laid in order to cross the terrain elevation and make it possible to connect these remote boxes to the central system.
In certain cases as well, the majority of the boxes or buoys are deployed in an area within which radio transmission is satisfactory, but access to them from the central command and recording system is still difficult when the central system is located outside of the above-mentioned zone. This also makes it necessary to lay a long transmission cable between these boxes and the central system.